The present invention relates generally to digital electronic circuits and, more particularly, to an apparatus for detecting a change of binary state of any one of a plurality of signals.
Keyless locks have become an increasingly attractive feature of new automotive vehicles in recent years. Using one such apparatus, the owner of a vehicle may remotely arm or disarm an alarm system, or operate the locks of the driver's side door, the passenger's side door or the trunk, by the actuation of the appropriate pushbutton switch on a small remote unit. A transmitter in the remote unit sends a digitally-coded RF sisal to a receiver in the vehicle, which responds in accordance the particular switch actuation. For convenience, the remote unit is ideally very small, desirably, of a size which can be attached to a keyring.
The need for compactness in such a unit necessitates its miniaturization in all aspects, particularly in the realm of power source, which cannot be expected to exceed the size of the lithium watch-type batteries. The concept of miniaturization extends also to efficiency in the use of integrated circuit chip area, which may impact significantly on the overall size of the remote transmitter unit.
In view of the extremely small battery capacity available to such remote unit, and further in view of a consumer's expectation that such unit shall operate for a matter of five years without battery replacement, it is clear that the unit must not said battery current when in its inactive state. From this, it is seen that the remote unit must include a wake-up circuit which activates the transmitter circuitry upon actuation of any of the switches. The concept of a wake-up circuit in this general type of application is well known, but the power constraints in this particular case drive the design to require that even the wake-up circuit must not draw current (other than device leakage) in its quiescent state.
As a consumer product operated from batteries, there is imposed on the wake-up circuit a further constraint of being capable of operating over a relatively wide range of voltages. Whereas, the nominal battery voltage may be 4.5 volts in order to operate with standard logic devices, it is anticipated that a fully-charged set may run as high as seven volts. Further, it is expected that the unit will operate as the batteries reach their end-of-life voltage, which may be as low as three volts. Hence, designing in a reasonable margin, the circuit should operate from supply voltages ranging from 3.0 volts to 9.0 volts. Thus, the area of the integrated circuit is a consideration since an overhead of fifty per cent is likely to be attributable to the use of high voltage devices.
By the very nature of its use, ordinary handling of such a unit imposes an additional constraint on its design. Keys are typically carried in pockets and purses. With such handling, it will be recognized that the remote transmitting unit described above might be wedged into a position such that one or more of the pushbuttons would inadvertently be held in an actuated position for an extended period of time. If this actuation were to drain power from the batteries, the unit might be rendered useless in a matter of hours. It is therefore an additional requirement on the design of the detecting circuit of such a unit that no power will be drawn from the batteries while the pushbuttons are in any quiescent state.
Of the families of integrated circuit devices currently available, it would appear that the requirements stated above lend themselves to the use of complementary metal oxide semiconductor (CMOS) devices. These devices can be fabricated with high densities, they work over a relatively wide range of voltages, and, most importantly, they draw zero dc current (apart from junction leakage current).
One way to implement such a detecting circuit using CMOS devices is to couple a monostable multivibrator (one shot circuit) to each switch input signal. The individual outputs of these one shots would be combined by an additional logic function to produce a single output signal representing a change in state of any one of the input signals. However, this approach may require an excessive area of the integrated circuit chip.
In view of the above, it is dear that there exists a need to develop an improved apparatus for detecting a change of binary state of any one from among a plurality of signals, which apparatus draws zero dc current from the power supply and which can be implemented on an integrated circuit chip in less area than is required by methods which may be currently known in the art.